1. Field of the Invention
This invention relates in general to aligning an RF transmitting signal between a vehicle related transmitting device and a vehicle related receiving device, and more specifically, to a remote vehicle accessory transmitter and a vehicle based control module for aligning the transmitting and receiving center frequencies.
2. Description of the Related Art
Transmitting devices such as remote keyless entry (RKE) fobs typically transmit data on a modulated signal to a receiving device such as a vehicle based remote keyless entry module. The signal is modulated on a carrier wave by the RKE fob at a respective center frequency and is received by the RKE module that is tuned to the same respective center frequency.
For two-way communication between a RKE module and a RKE fob, communication from the RKE module to the RKE fob is often limited in range. This is primarily due to the limited size of the antenna packaged within the RKE fob and the limited power supply of the RKE fob. Antennas that are small in size as that of the RKE fob combined with the RKE fob's limited power (i.e., small power supply) results in low gain, thereby limiting the reception range of the RKE fob. In addition, the power level emissions of RF transmitted signals are limited, as the Federal Communications Commission (FCC) maintains regulations on the maximum emission that may be generated by respective transmitted RF signal for a respective application.
To optimize a long-range signal transmission from the RKE module to the RKE fob having low gain, the bandwidth of the RKE fob for receiving a transmitted signal can be narrowed. The greater the distance of signal transmission between the transmitting and receiving devices, the narrower the bandwidth can be to receive the signal. Narrowing the bandwidth too much will not allow the received signal to fall within the receiver bandwidth if the transmitter and receiving center frequencies are in a mis-alignment condition. Thus, it is important to maintain the alignment of the center frequencies between the transmitter and the receiver when transmitting long distances. Typically, the transmitter and receiver are calibrated to a specific center frequency where a balance is maintained between the allowable distance that a RF signal is transmitted and the allowable width that a bandwidth may be narrowed given the maximum allowable transmitting distance.
Under certain conditions, such as temperature changes, the center frequency of the transmitting device may shift. Small shifts in the center frequency are typically tolerated by the receiving device due to the receiving device having a sufficient bandwidth for receiving the signal with small center frequency shifts. This allows for small discrepancies in the alignment of the center frequencies between the transmitting device and receiving device due to environmental changes or possible circuit tolerances. Devices such as RKE modules and RKE fobs typically are permanently tuned to a respective center frequency for transmitting and receiving signals, and as stated earlier, the RKE fob may have a small bandwidth for receiving signals from the RKE module. Maintaining a small bandwidth at a permanently tuned center frequency make the transmitting system susceptible to the issues described above. Even if the RKE module and RKE fob were tunable, a method would be required to calibrate the center frequencies of the two devices. Requiring the operator to knowingly and constantly calibrate the two devices would be burdensome.